The present invention relates to an oxide superconductor wire material through which it is required to pass higher current, such as magnet or power transmission cable; a method for producing the wire material and a method for jointing the wire material together. More specifically, the present invention may be applicable widely to oxide superconductor wire materials for use in higher system communication current as high as 100 A or more utilizing the superconducting phenomenon of oxide superconductors, including system structures such as superconducting power transmission cables, bus bars, long conductors, permanent current switch devices, large magnets, nuclear magnetic resonance systems, medical MRI systems, superconducting power reserving systems, magnetic separators, single-crystal drawing-up systems in a magnetic field, freezer cooling superconducting magnets, superconducting energy reservoirs, superconducting generators, nuclear fusion reactor magnets, accelerators, and current leads.
As superconducting materials in practical use, metal superconductors such as NbTi and Nb.sub.3 Sn have been known, and the superconductive joint technique for wire materials from these superconductors has been established. As to the technique of jointing oxide superconductors together, alternatively, a variety of methods have been proposed up to now. For example, there have been known
1. Japanese Patent Laid-open No. Hei 1(1989)-24379; PA0 2. Japanese Patent Laid-open No. Hei 1(1989)-17384; PA0 3. Japanese Patent Laid-open No. Hei 3(1991)-242384; and PA0 4. Japanese Patent Laid-open No. Hei 3(1991)-254473.
However, the known jointing methods have the following problems, and therefore, all of them are insufficient. Firstly, the orientation and density of superconductors produced by the methods 1 and 2 are so low, because the joint parts thereof are produced through solid reaction, that a sufficient critical current density is hard to procure.
According to the methods 3 and 4, use is made of Bi-2212 oxides, to produce a dense matrix in orientation by partial melt solidification. Because Bi-2212 crystal is produced by melting the oxide and then crystallizing the resulting oxide, a larger oriented crystal grows, advantageously for superconductive joints. For crystallization from the melted state, a method comprising inserting an intervening substance is generally selected. According to the method 3, for example, a precursor or a calcined body in powder, is arranged as the intervening substance in the joint part, followed by thermal treatment in the atmosphere. According to the method, however, the thermal treatment temperatures for the oxide superconductors and the calcined powder are different under some conditions because their melt temperatures are different by about 10 degrees, so good jointing can be attained only with much difficulty. If the intervening substance is singly melted, the crystal growing in a liquid phase is hardly connected satisfactorily to Bi-2212 as a connecting matter.
In recent years, thus, a method comprising integrating together such intervening substance and a connecting matter and then crystallizing the resulting integrated substance, has been investigated. According to the method 4, for example, an oxide superconductor primarily containing Bi-2212 is partially melted under heating at a state such that a calcined oxide powder with the same composition as a calcined substance is interposed at a part of the region of jointing together the oxide semiconductors. According to the method, however, the melt temperature readily varies depending on the ratio of the calcined powder mixed to the Bi-2212 phase; and the composition is assumed to be a single-core wire material. Thus, attention has not been paid to a method for jointing together a multi-core wire material, which is now under rapid technical development. For example, a high-performance tape wire material recently developed through energetic investigations by the present inventors is currently one of the wire materials with the highest critical current, among oxide superconductor wire materials. Details thereof are described for example in Japanese Journal of Applied Physics, vol. 34, page 4770-4773, 1995. For jointing a 55-core wire material of a representative tape thickness and shape, for example a thickness of 0.1 mm and a tape width of 5 mm, the superconductor therefor should comprise a single Bi-2212 phase and should have a superconductor core thickness of about 5 to 10 microns. Therefore, practically, it is very difficult to conduct the procedure of uniformly inserting intervening substances in the individual cores, as in the invention of 4, and additionally, no satisfactory performance is brought about, disadvantageously, if a Bi-2212 phase containing a calcined substance is used as the filling powder. Because the crystal nucleus is readily generated, starting from the intervening substance as the starting point, in such multicore wire materials during melt solidification, furthermore, crystal orientation is reduced at the joint part with addition of an intervening substance, compared with the orientation within the wire material, leading to the reduction of Jc at the joint part, disadvantageously. Such phenomenon is a specific phenomenon for jointing via melt solidification using a multi-core wire material in particular. The reason resides in that the multi-core wire material has a cross-sectional structure such that the silver and the superconductor are alternately laminated to each other, while the layer of the intervening substance does not have such multi-core structure.